In concrete slip forming machines used for building roads or the like, it is customary to form joints therein at predetermined intervals so that the elements such as freezing and thawing of the pavement can be accommodated without major damage to the roadway. These dowel bars are typically inserted all the way across the pavement being formed and are generally aligned with the forward movement of the machine. These joints can be perpendicular to the slab and to the direction of forward movement of the machine or they can extend across the strip of concrete being formed at somewhat of an angle thereto, which is commonplace in present day road construction so that each set of tires on a vehicle does not hit the joint at precisely the same time, thereby lessening the thumping problem that sometimes occurs when the concrete joints are perpendicular to the direction of movement of the vehicle thereon.
The aforementioned concrete slip forming machines have for many years used dowel bar insertion mechanisms thereon, for example like those shown in U.S. Pat. Nos. 4,798,495 and 4,799,820, both to Laeuppi et al and both of which are incorporated herein by reference.
Another common way to construct a dowel bar inserting apparatus is to utilize a trolley which is loaded with dowel bars on one side of the machine as it moves forward and then the trolley is moved transversely across the machine along a dowel bar placement pan subframe. As the trolley moves along the subframe, a cam device is actuated once for each place that a dowel bar retaining mechanism is disposed across the subframe. This cam operated device consequently pushes the lowermost one of the dowel bars in the trolley upwardly and outwardly into a respective dowel bar retaining mechanism. Once the subframe reaches a position where the joint in the concrete is to be formed, a pair of forked vibrating members for each dowel bar is actuated to push each of the dowel bars downwardly past the dowel bar holding mechanisms and into the concrete itself. After that, the dowel bar retaining mechanisms disposed across the subframe must again be loaded in the same fashion as discussed above wherein the trolley is manually filled with dowel bars from one side of the machine and then the trolley is moved across the subframe so that each of the dowel bar retaining mechanisms across the subframe will again receive a dowel bar as the cam operated device is actuated, one for each dowel bar retaining mechanism as the trolley moves across the subframe.
In this cam operated prior art device, the trolley needs to be loaded from only one side and it will distribute the dowel bars loaded thereon only in one direction of movement of the trolley across the subframe.
After each set of dowel bars is inserted into the concrete at each joint, the paving machine is provided with a mechanism for smoothing out the place where the dowel bars have been inserted. At some later time, a groove is sawed into the joint above where the dowel bars were inserted to complete the joint in the pavement. Each joint is formed using the same procedure so the trolley must be filled again each time, and as the trolley is made to travel across the machine transverse to its forward travel, the dowel bars need to be distributed into the respective dowel bar holders as described above.
The major problem with these prior art structures has been that the cam operated system of ejecting dowel bars from the trolley is slow and has not been one hundred percent reliable. Additionally, the moving parts of the cam operated ejecting system are subject to wear and consequently need to be repaired or replaced more frequently than is desirable. Accordingly, there is a need for an improved method and apparatus for positioning dowel bars for insertion into concrete in a slip forming paver.